This invention relates to air separation membranes and more particularly to a new type of asymmetric polyimide membrane with high permeances for air separation.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including N2 enrichment from air, carbon dioxide removal from natural gas and from enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate spiral wound polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. High performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne), and polytriazole have been developed to improve membrane selectivity, permeability, and thermal stability. These polymeric membrane materials have shown promising intrinsic properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8).
The membranes most commonly used in commercial gas and liquid separation applications are asymmetric polymeric membranes and have a thin nonporous selective skin layer that performs the separation. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as permeability or PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
One of the components to be separated by a membrane must have a sufficiently high permeance at the preferred conditions or extraordinarily large membrane surface areas is required to allow separation of large amounts of material. Permeance, measured in Gas Permeation Units (GPU, 1 GPU=10−6 cm3 (STP)/cm2 s (cm Hg)), is the pressure normalized flux and equals to permeability divided by the skin layer thickness of the membrane. Commercially available gas separation polymer membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. However, it is very complicated and tedious to make such asymmetric integrally skinned membranes having a defect-free skin layer. The presence of nanopores or defects in the skin layer reduces the membrane selectivity. Another type of commercially available gas separation polymer membrane is the thin film composite (or TFC) membrane, comprising a thin selective skin deposited on a porous support. TFC membranes can be formed from CA, polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. Fabrication of TFC membranes that are defect-free is also difficult, and requires multiple steps. Yet another approach to reduce or eliminate the nanopores or defects in the skin layer of the asymmetric membranes has been the fabrication of an asymmetric membrane comprising a relatively porous and substantial void-containing selective “parent” membrane such as polysulfone or cellulose acetate that would have high selectivity were it not porous, in which the parent membrane is coated with a material such as a polysiloxane, a silicone rubber, or a UV-curable epoxysilicone in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids. The coating of such coated membranes, however, is subject to swelling by solvents, poor performance durability, low resistance to hydrocarbon contaminants, and low resistance to plasticization by the sorbed penetrant molecules such as CO2 or C3H6.
A particular use for gas separation membranes is in air separation such as in the nitrogen generation systems (NGS) needed to provide fuel tank inerting for commercial and military aircraft. Fuel tank inerting is the process of replacing potentially flammable gas in the space above the fuel in the fuel tank with a non-flammable atmosphere. Useful membranes for separating oxygen or nitrogen from air must have sufficient selectivity to distinguish between these similar sized gas molecules and must also have high permeance. Since permeance determines the size and weight of the air separation module and selectivity determines the purity of the product gas. Normally, air separation membranes are in the form of hollow fiber and are formed into hollow fiber modules. Hollow fiber polymer membranes used for gas separations particularly for air separation have the advantages of low cost, high area packing density, good flexibility, and self mechanical support. However, fabrication of hollow fiber membranes with both superior permeability and selectivity is always a challenge due to the complexity of spinning process. Hollow fiber polymer membranes with integrally skinned asymmetric membrane structure are normally fabricated by a dry-wet phase inversion technique. There are three major steps including spinning dope preparation, spinning, and coagulation (or phase inversion) for the formation of hollow fiber membranes using this technique. Chung et al. reported that air-gap distance and elongational stress played important roles on the hollow fiber formation and the mass-transfer fluxes and spinodal decomposition in Markoffian and Onsager's thermodynamic systems. Work reported by Chung et al. also suggested that the main parameters affecting hollow fiber formation are rheological properties of spinning solution, bore fluid chemistry and flow rate, external coagulant chemistry, spinning dope chemistry and flow rate, fiber take-up rate, shear stress within an annular orifice of the spinneret, spinneret design parameters, and spinneret temperature. See Chung, J. MEMBR. SCI., 1997, 126, 19; Chung, Teoh, J. MEMBR. SCI., 1997, 130, 141; Chung, Hu, J. APPL. POLYM. SCI., 1997, 66, 1067.
US 2006/0011063 disclosed a gas separation membrane formed from polyetherimide by extruding a hollow fiber using a core liquid. For the described membrane, like other asymmetric hollow fiber membranes, one polymer solution is spun from an annular spinneret and the core liquid is pumped into the center of the annulus.
US 2008/0017029 A1 disclosed an asymmetric hollow-fiber polyimide gas separation membrane, an improved tensile elongation at break of 15% or more as a hollow-fiber membrane itself, an oxygen gas permeation rate of 40 GPU or more and a gas ratio of permeation rate of oxygen to nitrogen of 4 or more measured at 50° C. In addition, this work taught an asymmetric hollow-fiber gas separation membrane obtained by heat-treating the asymmetric hollow-fiber gas separation membrane at a maximum temperature of from 350° to 450° C. The asymmetric hollow-fiber gas separation membrane has sufficient mechanical strength even after the heat-treatment at a maximum temperature of from 350° to 450° C.
US 2009/0297850 A1 disclosed a hollow fiber membrane derived from polyimide membrane, and the polyimide includes a repeating unit obtained from aromatic diamine including at least one ortho-positioned functional group with respect to an amine group and dianhydride.
U.S. Pat. No. 7,422,623 reported the preparation of polyimide hollow fiber membranes using annealed polyimide polymers, particularly polyimide polymers with low molecular weight sold under the trade name P-84. The polyimide polymers are annealed at high temperature from 140° to 180° C. for about 6 to 10 hours to improve the mechanical properties of the polymers. The resulting membranes prepared from the high temperature annealed polyimides are suitable for high pressure applications. This polymer annealing method, however, is not suitable for high molecular weight, easily thermally crosslinkable, or easily thermally decomposed polymer membrane materials.
The present invention provides a new type of polyimide hollow fiber and flat sheet membranes with high permeances for air separation and a method of making these membranes.